The intracellular effects of 1,25-dihydroxyvitamin D3, a steroid hormone, are mediated by a specific receptor protein which, on complexing with hormone, interacts with the cell nucleus inducing specific gene expression necessary for intestinal calcium and phosphate absorption. Preliminary observations in our laboratory indicate that the 1,25(OH)2D3 receptor may differ from other steroid receptors in that a temperature dependent step is not required for conversion to the nuclear binding form. Instead, exposure to hormone at 4 C increases the affinity of receptor for DNA in vitro while heating (25 C, 30 min) rapidly and irreversibly inactivates the nucleic acid binding site. Furthermore, we have obtained evidence for an interaction between this receptor and RNA. Studies with synthetic and natural RNAs and DNAs suggest that receptor:RNA interactions are quite specific and may be of physiological significance. The objectives of this proposal are: 1) To establish whether receptor is converted to an activated, nuclear binding form on binding 1,25(OH)2D3 at 4 C and examine the molecular basis for changes in receptor accompanying hormone binding and heating using the following parameters--binding to purified nuclei, chromatography on DNA-cellulose, phospho-cellulose and DEAE-cellulose and isoelectric focusing; and 2) To assess the physiological significance of RNA:receptor interactions by studying the molecular specificity of RNA versus DNA binding using synthetic polyribo and polydeoxyribonucleotides as well as purified RNA classes from target and non-target tissues.